The present invention relates to an optical sensor as well as a method for producing the same, and its use.
Optical sensors for determining the carbon monoxide content in the air are used, among other things, in fire alarms. Their function is based on the fact that a layer sensitive to carbon monoxide changes color reversibly upon contact with the gas to be ascertained. This change of color is recorded by a detector, and when a predetermined minimum concentration is exceeded, an alarm is triggered.
In U.S. Pat. No. 5,618,493 such a carbon monoxide sensor is described. It includes two substrates to which sensitive layers are applied. One of the layers is provided especially for low humidity and low air temperature, the other for a higher temperature and humidity. In order to be able to detect the color change of the sensitive layers at contact with carbon monoxide, a light-emitting diode is provided as radiation source in the fire alarm described, whose light passes through the sensitive layers. The absorption appearing during this process is ascertained, using a photodiode. However, such a sensor does not have the sensitivity suited to strict standards, and has a long response time.
The present invention is based on the object of making available a carbon monoxide sensor having great sensitivity and a short response time.
The optical sensor according to the present invention has the advantage that it has a very short response time and very high sensitivity to carbon monoxide. This is achieved by adding a transition metal compound to the sensitive layer of the sensor, which reversibly undergoes chemical bonding with CO, and thus actively brings carbon monoxide in the air to the sensitive layer. This makes possible the rapid reaction of the sensor to changing carbon monoxide concentrations, but also a lowering of the response threshold of the sensitive layer. The sensitive layer is preferably deposited as a thin film on a translucent substrate.
As a transition metal compound, copper(I) chloride is suitable above all, since it rapidly and effectively forms complexes with carbon dioxide at room temperature.
In a particularly advantageous embodiment, the sensitive layer includes a matrix of hydrogel, since carbon monoxide has a substantially higher diffusion constant and solubility in aqueous media than in the usual polymer layers. This further raises the sensitivity and lowers the response time.